1. Field of the Invention
The present invention relates to an electronic endoscopic apparatus. Particularly, the present invention relates to an electronic endoscopic apparatus for diagnosing a living body mucous membrane by producing a spectral image of the living body mucous membrane.
2. Description of the Related Art
Conventionally, an electronic endoscopic apparatus (Narrow Band Imaging-NB1) that produces a spectral image for diagnosis is well known as an electronic endoscopic apparatus using a solid-state imaging device. In the electronic endoscopic apparatus (Narrow Band Imaging-NB1), narrow-band spectral images of a living body mucous membrane of the digestive organ, such as the stomach, are obtained by imaging the living body mucous membrane through a narrow-band-pass filter, which transmits light only in a narrow wavelength band. Then, the narrow-band spectral images or the like are synthesized to produce a spectral image for diagnosis. This electronic endoscopic apparatus includes a rotation filter and performs imaging by using a plane-sequential method. The rotation filter is a filter formed by combining three kinds of narrow-band-pass filters, each transmitting light in a wavelength band different from each other. In the electronic endoscopic apparatus, the living body mucous membrane is imaged while the living body mucous membrane is sequentially illuminated with light that has been separated by being transmitted through each of the narrow-band-pass filters. Accordingly, narrow-band spectral images of the living body mucous membrane are obtained. The spectral image for diagnosis of the living body mucous membrane, which is obtained as described above, can represent a very fine structure of the living body mucous membrane, which could not be represented in a conventional apparatus.
Meanwhile, regarding an electronic endoscopic apparatus that performs imaging by using a plane-simultaneous method by placing an RGB mosaic filter, which is used in ordinary color image photography, in a solid-state imaging device, a method for obtaining an image that has a quality similar to that of the narrow-band spectral image, obtained through the narrow-band-pass filter, has been proposed. In this method, the image is obtained by performing an operation based on color image data obtained by imaging a living body mucous membrane.
The method, as described above, has been proposed by finding out a fact that spectral reflectances of the living body mucous membrane in the entire wavelength band of visible light can be substantially restored using three major components, namely the first major component through the third major component. This fact was found out by performing major component analyses for estimating the spectral reflectance of the living body mucous membrane using a multiplicity of sets of measurement data on the spectral reflectances of the living body mucous membrane in the wavelength band of visible light. In this restoration method, it is possible to obtain an image having a quality similar to that of the narrow-band spectral image in a pseudo manner by performing an operation using spectral reflection estimation matrix data and image data of each of RGB colors that has been obtained by performing imaging through an ordinary RGB mosaic filter corresponding to the three major components. The spectral reflection estimation matrix data is data obtained in advance using a multiplicity of sets of measurement data on the spectral reflectance of the living body mucous membrane (please refer to Japanese Unexamined Patent Publication No. 2003-93336 and “Analysis and Evaluation of Digital Color Image”, Yoichi Miyake, University of Tokyo Press, pp. 148-153).
The spectral reflectance of a living body mucous membrane in a short wavelength band is lower than that of the living body mucous membrane in a long wavelength band, as illustrated in FIG. 4. Therefore, when reflection light reflected by the living body mucous membrane is received by an imaging device to form an image, the light receiving level of light (for example, green light and blue light) in a short wavelength band is lower than the light receiving level of light (for example, red light) in a long wavelength band. Meanwhile, an image signal obtained by receiving light of each of the colors includes a noise component, and a constant noise component is included in the noise component. The constant noise component is included at a substantially constant ratio with respect to a maximum light receiving level of light receivable at the imaging device. Specifically, when a spectral image is obtained by reading the image signal, a constant noise component is included in the spectral image at a constant ratio with respect to the maximum light receiving level regardless of the light receiving level. Therefore, the ratio of a constant noise component included in a spectral image corresponding to the short wavelength band is larger than that of a constant noise component included in a spectral image corresponding to the long wavelength band, thereby the quality of the spectral image corresponding to the short wavelength band becoming lower. Hence, there is a demand for reduction of the ratio of the constant noise component included in the spectral image corresponding to the short wavelength band.
The ratio of the constant noise component included in the spectral image corresponding to the short wavelength band might be reduced by increasing the light receiving level corresponding to the short wavelength band when an image is obtained by the imaging device. However, if the light receiving level corresponding to the short wavelength band is increased, there is a problem that the light receiving level corresponding to the long wavelength band is saturated and exceeds the maximum light receiving level.